Bloody German atrocities against Belgian civilians at the outset of World War I still haunt gentle towns such as Dinant and Louvain whose sacrifice sparked global outrage and a drive to curb war crimes.

Seeking a quick, knockout victory, German armies ploughed through neutral Belgium aiming to take Paris from the north but unexpectedly fierce resistance at Liege in the east and then Namur, south of Brussels, threw their timetable out of the window.

By mid-August 1914, their advance troops reached Dinant, below Namur where the River Meuse has carved a deep ravine through the chalk hills, making the small ancient town a key crossing point.

There they ran into French troops, coming into battle at the request of the Belgian government. Among their number was a certain Charles de Gaulle, wounded in fighting that largely spared the inhabitants.

A week later, 30,000 German troops tried to breach French defences and again the town's inhabitants saw no immediate threat despite the intensity of the fighting.

But for the now exhausted German troops, there was a growing sense of frustration as casualties mounted without any sign of the planned breakthrough.

Rumours that Belgian civilians have taken up arms to "shoot them in the back" or that Belgian nurses are mistreating their wounded - cutting off their hands and gouging out their eyes - quickly gained credence and turned to murderous intent.

Civilians targeted

Early on Aug 23, the Germans took 43 men being held at the Leffe abbey on the outskirts of Dinant and lined them up facing a wall.

"They were shot in the back because the Germans considered them to be irregulars" and not soldiers covered by military convention, said Michel Coleau, Dinant's archivist.

Those killings were only the beginning.

Late that afternoon, 150 men were lined up in four rows facing a double line of German soldiers.

"A lieutenant colonel mounted on a horse gives the order for the summary execution. One hundred and sixteen men fell," Coleau recounts.

A monument on the site still denounces "Teutonic fury" despite a formal reconciliation with Germany sealed in 2001.

Later the same day, German soldiers panicked under French artillery and opened fire at point blank range on a group of men, women and children who had sought shelter, killing 77.

"The youngest victim was a baby girl of just three weeks, finished off with a bayonet," Coleau said, with the killings joining a long list of "massacres of the innocents."

In all, nearly 650 civilians died that day at the hands of German troops.

The following morning, the Germans set fire to 750 of the some 1,000 buildings in Dinant, including its cathedral, squeezed in between the river and ravine face.

One hundred years later, giant photographs of the scene put up around the town show the scale of the disaster.

Further north in the famous medieval university town of Louvain, the same fury seized the German soldiers.

On August 25, they killed nearly 250 civilians and burned down 2,000 buildings, including the university's world renowned library where some 300,000 manuscripts and ancient works were destroyed.

"It marks the beginning of 'total war' which also targets civilians and their culture," Louvain archivist Mark Derez said.

"The Germans said it was a punishment ... but I believe it was an organized terror attack to intimidate the civil population, to terrorize recently occupied areas and to sap the morale of the Belgian army," Derez said.

Combined with other recorded atrocities such as in Vise, Tamines, Andenne and Termonde, Belgium counts some 6,500 civilian deaths at the hands of German soldiers which created "a terrible fear" in the country, said Sophie Soukias, specialist World War I researcher at the CEGESOMA institute in Brussels.

News of the killings was quickly taken up by the allied powers, especially Britain, which used them to help justify its intervention in the war and to encourage other countries such as the United States to follow suit in a crusade to save civilization from German militarism.

They also sparked outrage that led in time to the idea that war crimes must be punished without fail.

"Today it seems natural to us that when there is a war crime, it has to be punished, made good, but at the time that was completely new," Soukias said.